1. Field of the Invention
The present invention relates, in general, to methods and systems of wireless networks, especially methods for reducing power consumption by stations of the network. More particularly, the invention relates to, but is not limited to, Power saving methods for wireless sensor and metering networks operating in the Sub 1 GHz band, especially networks using the emerging IEEE standard 802.11ah.
2. Relevant Background
There is a clear need to save power in devices in communications networks and, particularly, in mobile devices in wireless networks. The particular cases of wireless networks implementing the IEEE 802.11 a/b/e/g/n standards have specified time windows in which the stations of the network can save power by entering a sleep mode, in which most of the components of a device are turned off according to a known schedule.
A more recent amendment to the 802.11 standard was developed in the 802.11v amendments. One of the features added therein was the WNM—sleep mode (Wireless Network Management). It extends the allowable sleep time for a STA before disconnect by AP.
However, the particular cases of wireless sensor and metering networks present several problems with using the known sleep modes. The data might only need to be transmitted infrequently. The sensors could have very stringent power saving requirements, needing even longer sleep periods. Most important, there could be a large number (e.g., 6000) of STAs in a sensor network; if too many were to wake at the same time, they could all contend for medium access (e.g., sending PS-Poll frames) at the same time. This could result in signal collisions, causing network inefficiency and wasting power at the STAs as signals are repeated.
In order to meet these challenges, the Institute of Electrical and Electronics Engineers (IEEE) organized a task group (TGah) to develop methods and standards for communication networks using radio transmissions in the Sub 1 GHz frequency range, 902 to 928 MHz. One advantage of this frequency range is that it allows greater range and suffers less interference from intervening objects. Another advantage is that by using the new frequency range, there is no need to maintain compatibility with previous standards, so the types and forms of the frames can be optimized for efficiency.
3. Glossary and Acronyms
As a convenient reference in describing the invention herein, the following glossary of terms is provided. Because of the introductory and summary nature of this glossary, these terms must also be interpreted more precisely by the context of the Detailed Description in which they are discussed.    ACK Acknowledgement    AID Association Identification    AP Access Point    APSD Automatic Power Save Delivery    ARP Address Resolution Protocol    BSS Basic Service Set    DMS Direct Multicast Service    DTIM Delivery Traffic Identification Map    E_TIM Extended TIM    FMS Flexible Multicast Service    MAC Medium Access Control    MPDU MAC Protocol Data Unit    PHY Physical signaling layer    TBTT Target Beacon Transmit Time    TIM Traffic Identification Map    WNM Wireless Network Management